The key difference is that a Raspberry Pi is a computer (albeit a small one) and an Arduino or compatible is a microcontroller.
In saying that, one is better for some tasks than another.
An Arduino can use a colour sensor, control servos and motors etc for the LEGO task, whereas cameras and image processing is better handled by a computer and thus (e.g.) a Raspberry Pi.

yes for aquarium project, Raspberry is good. But for Lego Bricks, go for Arduino as suggested by andrew.
I am a fan of Lego bricks game, and I am creating a project where, a robot will find the same color LEgo bricks and make tower of them by putting them over one another. I have used Arduino to find the color of the bricks with help of below project:color detection using arduino
now I am using servo motors to pick up the same color bricks but not succeeding, so the point of this whole talk is that, as this thread is already 6months old, so have you created such system, where you can keep the same color bricks aside? Please share that how you are putting them in different buckets, means using any motor/robot/or any other technique.
Thanks.